Yeast RNase III as a key processing enzyme in small nucleolar RNAs metabolism

The variety of biogenesis pathways for small nucleolar RNAs (snoRNAs) refle cts the diversity of their genomic organization. We have searched for yeast snoRNAs which are affected by the depletion of the yeast ortholog of bacte rial RNase III, Rnt1. In a yeast strain inactivated for RNT1, almost half o f the snoRNAs tested are depleted with significant accumulation of monocist ronic or polycistronic precursors. snoRNAs from both major families of snoR NAs (C/D and H/ACA) are affected by RNT1 disruption. In vitro, recombinant Rnt1 specifically cleaves pre-snoRNA precursors in the absence of other fac tors, generating intermediates which require the action of other enzymes fo r processing to the mature snoRNA. Most Rnt1 cleavage sites fall within pot entially double-stranded regions closed by tetraloops with a novel consensu s sequence AGNN. These results demonstrate that biogenesis of a large numbe r of snoRNAs from the two major families of snoRNAs requires a common RNA e ndonuclease and a putative conserved structural motif. (C) 1998 Academic Pr ess.